In general, a fuel cell is an electrochemical device in which electrochemical reaction takes place between fuel gas, such as gas containing hydrogen, and oxidizer gas, such as air containing oxygen, in an electrolyte layer to directly extract electric energy. Depending on a kind of electrolytes, the fuel cell is typically classified into Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC) and Polymer Electrolyte Fuel Cell (PEFC).
Among these, due to various reasons, such as an ease of handling because of the use of an electrolyte composed of a solid oxide with an oxygen ion conductivity, high electric power generating efficiency, waste heat at high temperatures and fuel gases available in a wide range, the solid oxide fuel cell receives expectations to be applied to power supplies for moving objects and on-site cogeneration systems.
Such a solid oxide fuel cell is classified into a cylindrical structure type and a flat-structure type on structural features. The cylindrical structure type includes a cylindrical electrolyte whose inner and outer surfaces are formed with electrodes, respectively. The flat-structure type includes a rectangular or circular flat-shaped electrolyte whose both surfaces are formed with electrodes, having a greater potential than the cylindrical structure type in a capability of obtaining a high-power-density configuration.
Further, in such a flat-structure type fuel cell, flat-shaped electric power-generating elements, each including an electrolyte layer with both surfaces laminated with a fuel electrode layer and an air electrode layer, and separators, doubling as interconnectors, each having one surface formed with fuel gas flow channels and the other surface formed with air flow channels, are alternately stacked, upon which a stack structure is applied with a large load in a stack direction to ensure gas sealing effects and electrical connections.
In addition, proposals have heretofore been made to provide an electrode support type structure wherein when forming the electrolyte layer in a thin film configuration in order to reduce electric resistance, either a porous fuel electrode or a porous air electrode is used as a support body on which an electrolyte layer and the other electrode layer are formed.
By the way, with the fuel cell, chemical energy resulting from oxidation of fuel gas is extracted as electric energy and remaining energy is consumed as heat. Also, Joule heat occurs due to internal resistance of the cell resulting from flow of current generated in an electric power generating cell. Since these developed heats occur in proportion to current densities on respective locations, the presence of an imbalance in current density on an electric power generating surface results in an imbalance in temperature on the electric power generating surface.
If the solid oxide fuel cell is comprised of an electric power generating cell composed of ceramics, heat conductivity is low with a resultant difficulty in alleviating an in-plane temperature difference in the electric power generating cell. Additionally, the use of separators made of ceramics causes a whole of the components parts to be structured with ceramics, resulting in a further increase in the in-plane temperature difference. Accordingly, fuel gas and oxidizer gas become expedient measures for cooling the electric power generating cell in which heat builds up due to electric power generation. Among these, oxidizer gas with a large flow rate takes a leading part.
In the meanwhile, on characteristics of the solid oxide fuel cell, the degree of oxygen ion movement increases in areas with high temperatures to result in a decrease in internal resistance and, hence, a fuel gas inlet portion, which originally has a large current density, takes a further increased current density with a resultant further increase in the in-plane temperature difference of the electric power generating cell. This results in a drop in a temperature of the oxidizer gas flow inlet portion to allow the fuel gas inlet portion to have a distribution pattern with increased temperatures, causing strain to occur due to thermal stress.
Japanese Patent Application Laid-Open Publication No. 2002-203579 (see page 9 and FIG. 1) discloses a fuel cell with a structure wherein as a countermeasure to prevent concentration distribution and temperature difference in gases that cause a temperature difference to occur in such an electric power generating surface, gas is preheated and blown out from multiple blowout ports toward a cell.